Keystone correction circuit



1945. c. E. HALLMARK KEYSTONE CORRECTION CIRCUIT Filed April 1, 1942 DOC INVENTOR HALLMARK Patented Jan. 9, 1945 Clyde E. Hallmark, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation,

a corporation of Delaware Application April 1, 1942, Serial No. 437,118

4 Claims This invention relates generally to a scanning system for television systems and more particularly to scanning circuitsfor correcting keystone distortion in such systems.

As is well known in the art, certain types of cathode ray tubes now in general use for converting optical images into electric signals entail the production of pictures having a keystone pat tern instead of the desired rectangular pattern. This so-called keystone effect results from the fact that the mosaic of the tube, in order to be supported perpendicularly to the optical system which focuses the picture to be transmitted, must be disposed at an angle to the axis of the electron gun. Thus, even though the beam is deflected horizontally through the same angle for each line of the scanning pattern on the mosaic, it actually traces shorter lines toward the bottom of the mosaic than at the top since the lower part of the mosaic is ata lesser distance-from the electron gun.

Keystone distortion may be corrected so that a scanning pattern is produced which is or substantially rectangular configuration by modulatmg the horizontal scanning wave in accordance with the instantaneous amplitude of the field scanning wave. -Thus, as a field is traced, the

field scanning wave modulates the horizontal scanning wave to gradually alter thelength of successive horizontal scanning traces so that the components of the field scanning frequency as well as the fundamental thereof. 1

Accordingly, the primary object of this inven tion is to provide a novel method and apparatus for obtaining keystone correction in television systems and for eliminating the distortion in such systems which is effected by keystone correction.

Another object of this invention is to provide a novel modulation circuit in connection with the horizontal scanning circuit of a television system for correcting keystone distortion.

keystone effect is compensated for and a substantially rectangular field is scanned.

While certain scanning systems of the prior art have accomplished keystone correction to a certain degree, they have failed to produce a rectangular scanning pattern wherein all scanning lines are of precisely the same length and/or entirely free from lengthwise displacement. As

a result, in such systems the picture fields reproduced at the-television receiver have had ragged vertical sides or the lower portions thereof have included lines of excessive length and/or lines which were displaced lengthwise. The scanning fields of these systems have been particularly distorted at their lower edges. These forms of residual distortion have been found to be the result of modulation distortion and of the failure of the systems to remove the higher order frequency components of the field scanning frequency from the modulated horizontal scanning wave. It is possible to eliminate the fundamental of the field scanning frequency from horizontal scanning voltages, but prior to the present invention no apparatus has been provided for eliminating the higher order frequency having substantially perpendicular sides and also A further object of this invention is to providea keystone correction circuit particularly adapted for use in connection with electroma netic scanning systems in cathode ray apparatus.

A still further object of this invention is to provide a method and apparatus for eliminating higher order distortions of the scanning fields in television systems.

There is provided in accordance with this invention a horizontal scanning control circuit comprising a scanning generator for generating simultaneous line scanning waves of equal amplitude but of opposite phase. These waves are simultaneously impressed on the input of a balanced modulator wherein their amplitude is modulated in accordance with the instantaneous amplitude of each field scanning wave. There is provided, in the output of the balancedmodulator, a choke coil wherein the output of the [balanced modulator is balanced to eliminate the field scanning frequency fundamental together with the higher order components thereof. The modulated line scanning waves are added in the balancing choke coil and impressed on the line scanning deflecting coils ofa cathode ray tube. The particular circuit and method of balancing the various components of the modulated horizontal scanning wave provides a picture field sides of straight line form so that there is no sensible vdistortion of the rectangular scanning field.

For a better understanding of the present invention, together with other and further oblects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims- Referring to the drawing: I Fig. 1 is a circuit diagram illustrating the preferred embodimentsof the invention;

Fig. 2 is a diagram illustrating keystone distortion together with the desired rectangular scanning field;

Fig. 3 is a diagram illustrating one type of distortion occurring in conventional keystone corrected scanning fields; and

Fig. 4 is a diagram illustrating distortion occurring in conventional keystone corrected scanning fields as a result of the presence in the horizontal scanning wave of higher order components of the field scanning wave.

In Fig. 1 of the drawing, there is illustrated a circuit for generating line scanning waves and a modulating circuit for introducing keystone transmitting electrical signals corresponding to an optical image. There is provided a conventional timer chain! for generating synchronizing pulses to control the horizontal synchronizing signal generator 2 and the field scanning signal generator 3. The output signal of horizontal synchronizing signal generator! comprises a series of square-top pulses having a frequency depending upon the number of lines it is desired to scan. It may be assumed for purposes of iilustration that the image will be scanned at the rate of 30 frames per second. The horizontal scanning will be of the order of 525 lines per frame, and, therefore, the horizontal scanning frequency will be of the order of 15,750 cycles per second.

The horizontal synchronizing pulse wave is converted into a saw-tooth scanning wave by the saw-tooth wave generating circuit 5. This circuit includes a first time-constant circuit comprising a condenser I and a resistor 8 which are connected in the anode circuit of the tube III. A second time-constant circuit comprising condenser l2 and resistor 18 is connected in the V correction, together with a cathode ray tube for cathode circuit of tube l8 and in series with the connected with the horizontal synchronizing generator 2 by means of condenser II and grid resistor [4. The grid of tube is normally biased to render it non-conductive.

The output of generator is impressed on the conductors l8 and II which are connected to the input circuit of a modulator 28 through coupling condensers 2'2 and 23. Modulator includes a pair of similar thermionic tubes and 28, the input circuits of which are connected to balanced input resistors 28, 29 and bias resistor 38. Resistors 3| are connected in series with the oathodes of tubes 25 and 28 for providing correction of the tube outputs'so that, in the event that the tubes 25 and 28 are slightly dissimilar, their outputs may nevertheless be closely balanced. This is in accordance with conventional practice as regards balanced modulators. The anodes of tubes 25 and 28 are shunted by a resistor 32, together with a balancing inductance element or choke coil 85. The: choke coil 85 includes a pair of matched windings 38 and 81 together with a tap between the two windings to which the anode operating voltage source of supply is connected. Resistor 32 is of such a value that the output impedance is substantially purely resistive for all appreciable components of the horizontal scanning .wave. A power amplifier 88 is coupled through a condenser to the output circuit of the balanced modulator 28 for converting the horizontals-scanning wave into a current wave. of the desired amplitude.

, A conventional cathode ray tube 4! may be connected with a suitable source of potential, not shown, for generating a cathode ray beam for scanning mosaic 4|. It is to be noted that mosaic 4| is supported at an angle to the axis of the electron gun of the tube in the well known manner so that the optical image may be focused directly on the mosaic. Thus, as mentioned hereinbeifore, the cathode ray beam normally would scan a field having a keystone configuration as distinguished from the desired rectangular shape.

In accordance with the present invention, however, the output of amplifier .38 is coupled with deflecting coils 42 of the tube 48 through transodes of modulator tubes 25 and 28 through conductor 46, connected as shown in Fig. 1.

In order to transmit electrical signals representative of an optical image, an optical system, not shown in the drawing, focuses the image on mosaic 4| whereupon a cathode ray beam generated by the electron gun of tube 48 is caused to trace successive fields of scanning lines by the windings 42 and 45.

The line scanning is controlled initially from timer chain I, the output of which consists of a series of timing pulses. Horizontal synchronizing generator 2, controlled by the timer chain L develops a series of square-top pulses, as illustrated in Fig. 1, the frequency of these pulses being of the order of 15,750 cycles per second.

Assuming the generator 5 to have reached its stable operation and to be in such condition that its output voltage wave is at its zero point, at this time the tube In is biased to cutoff and conditions are such that condenser l is gradually charging from anode supply B+ through resistor 8 and the voltage across this resistor therefore is gradually increasing in amplitude. At thesame time condenser I2 is discharging at a gradual rate through resistor l3. The rate of charge and discharge of these condensers depends, of course, upon the time-constant of their respective circuits. Since time-constant circuits 1, 8 and l2, l3 are identical, the voltages across resistors 8 and I 3 will be respectively increasing and decreasing at precisely the same rate. This change of voltage continues until'the timing pulse from the horizontal synchronizing generator occurs, whereupon the bias on the grid of tub II is changed to render the tube conductive. Condenser is substantially instantaneously discharged through tube l8 and condenser I2 is simultaneously instantaneously charged. Hence.

there appears across the resistors 8 and I3 simultaneous saw-tooth waves of equal amplitude and opposite polarity. These saw-tooth waves are of a frequency controlled by synchronizing generator 2, and therefore, are of the order of 15,750 cycles per second.

The saw-tooth voltages are applied across resistors 28 and 28 of modulator 28 and are impressed on the grids of tubes 25 and 25 andsince they are of opposite polarity and of exactly the same shape and amplitude, the inputs of tubes the field scanning generator 3 is impressed on the cathodes of tubes 25 and 26 in parallel by way of conductor 46, and therefore, the horizontal scanning waves are modulated to have gradually increasing amplitudes in accordance with the increasing amplitudes of the field scanning Waves during each field scanning period. The outputs of tubes 25 and 26 consist of the line scanning waves in opposite polarity together with field scanning waves components in phase with one another. Hence, in the opposing windings 36 and 31 of the choke 35,the line scanning waves are added while the field scanning wave components thereof oppose each other and are Not only is the fundamental of cancelled out. the field scanning wave cancelled out' but'also the higher order frequency components, the choke 35 having exactly the same number of turns in each of its windings 36 and 31. Furthermore, if tubes 25 and 26 are not identical as to their characteristics, resistors 3| largely counteract any unbalance in output from this source and in addition render an improvement in modulation linearity. The use of the balancing choke coil 35 provides a single ended output for modulator 2D, and therefore, the modulated line scanning wave may be coupled through condenser 44 to power amplifier 38. Power amplifier 38 converts the output of modulator 20 into a saw-tooth current wave of the desired amplitude. The output of amplifier 38 is translated by transformer 39 and impressed across deflecting windings 42, under the influence 'of which the cathode ray beam of tube 40 is deflected across the mosaic 4|. Picture As pointed out hereinbefore, resistor 32 is of such a value that the balanced output impedance of the modulator is substantially a pure resistance and, therefore, the output circuit of the modulator cannot have resonance characteristics which might introduce distortions in the horizontal scanning waves. waves, modulated during each field, are of increasing amplitude as the beam scans successive lines from the. top to the bottom of the mosaic, the scanning field is rectangular, as shown in Fig. 2 at a, b, c and d. If the scanning waves are not modulated to correct their amplitude, the field scanned by the beam in tube 40 would have a keystone shape such as indicated by e, f; y and h in Fig. 2. Furthermore, the precisely balanced saw-tooth wave generator and modulator circuit shown in Fig. 1 provides a straight edge scanning field and prevents the scanning field from having .an irregular form, such as shown in Figs. 3 and-4 wherein certain lines, as scanned in the mosaic 4!, are lengthened or displaced horizontally as a result of modulator unbalance.

The balanced choke coil 35, as provided in accordance with the present invention, is particularly adapted to eliminate the higher order frequency components of the field scanning frequency which appear in the modulated line scanning wave. As described hereinbefore, the coil v is wound in such a manner that the turns of each of the windings 36 and 31 lie side by side and adjacent one anothen throughout their length. Thus, there is extremely close coupling between windings 36 and 31 and minimum leakage reactance therebetween. If there were a substantial leakage reactance, it would appear as. an

impedance in series with winding 36 and there.

would be a voltage drop across it which would be impressed on coupling condenser 44. There- Since the horizontal scanning fore, the higher order frequency components of the field scanning wave in the modulated line scanning waves would create a distortion voltage, corresponding to the higher order components, across the input circuit of power amplifier 38, and distortion would occur in the field scanned by the cathode ray of tube 40. However, the provision of the' closely coupledchoke coil 35 substantially-eliminates the leakage reactance effect and also provides a substantially perfectly baltherein is such that the windings cannot be as closely coupled as those of a choke coil, such as is provided in this invention. Therefore, the transformer would introduce asubstantial leakage reactance in circuit with the outputcircuit of modulator 20 and consequently there would be an ap reciable higher order frequency voltage in that 'rcuit which would introduce distortion in the input circuit of amplifier 38 and in the field traced by the cathode ray. From this it is obvious that choke coil 35 provides a very definite advantage over a center-tapped transformer or" any other form of coupling wherein leakage reactance occurs.

The various circuit elements of the generator and modulator shown in Fig. 1 have been indicated as having certain constants for purposes of illustrating a workable form of the invention, but it is not intended that the invention should be limited to these specific constants, as it will be obvious to those skilled in the art that the invention is applicable to circuits having entirely different constants for providing various numbers of scanning lines and frames per second. It will also be obvious to those skilled in the art that the invention may be applied to various types of cathode ray tubes wherein keystone correction is necessary.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and .modifications as fall within the true spirit and prising a pair of opposed windings each of whichis connected to one of the anodes of said devices whereby low-frequencycomponents of saidmodulated voltages are balanced out, and means whereby the voltages appearingacross said choke coil are applied to a pair of deflection windings of a cathode ray tube.

2. In combination, a pair of vacuum discharge devices connected in push-pull relation, a source of high-frequency pulses in the input of said de-'- vices adapted to produce simultaneous saw-tooth voltages of equal amplitudes and opposite polarities on the grids of said devices, a source of low odes of said devices for modulating the high-trequency pulses, a balanced choke coil connected across the anodes of said devices to balance out the low-frequency components or said modulated voltage, and a load circuit coupled to said choke coil.

4. In combination, a pair of vacuum discharge devices connected'in push-pull relation, a source of high-frequency pulses coupled to the input circuits of said devices, a source of low-frequency saw-tooth voltages connected to the cathodes of 3. In combination, a pair of vacuum discharge 5 devices, a source of high-frequency pulses coupled to the input circuits of said devices, a source of low-frequency pulses connected to the cathsaid devices for modulating the high-frequency pulses, means including a choke coil connected across the anodes of said devices to balance out the low-frequency components and their higher order components in said modulated voltage, and a load means coupled to said means.

CLYDE E. HALLMARK. 

